1. Field of the Invention
The present invention relates to an engine starting device, and more particularly, to an engine starting device which is suitable for preventing a starter motor from being rotated together with an engine by a driving force of the engine when the engine revolution number is increased after ignition of the engine is started.
2. Description of the Related Art
In an engine starting device, a starter motor used for cranking an engine is controlled such that the revolution number is converged to a substantially constant target revolution number, and drives the engine to ignite the engine. Therefore, after the ignition is started, as the engine revolution number is increased, the target revolution number relatively becomes lower than the engine revolution number. Therefore, if the starter motor is kept connected with the engine even after the engine been ignited, the starter motor receives a driving force from the engine and is rotated, and the starter motor is rotated together with the engine. As a result, the starter motor becomes a load, which interferes with rotation of the engine.
In order to prevent the starter motor from rotating together with the engine, there is a method that after the ignition is started, meshing of gears which connect the starter motor and the engine is released or a clutch provided between the starter motor and the engine is disengaged. In a system using the starter motor as a generator, a so-called generator-motor driven by the engine after the start of the engine, the engine and the starter motor, that is generator can not mechanically be separated from each other even after the ignition is started. As disclosed in Japanese Patent Application Laid-Open No. H3-3969, supply of excitation current of the starter motor is stopped after the ignition is started.
However, the revolution number at which it can be reliably judged that the engine operation is shifted to independent or self-driving operation is much higher than the cranking revolution number. Therefore, if excitation of the starter motor is stopped at an early stage during the increase in the revolution number after the engine ignition is started, complete explosion state can not be obtained and the start of the engine is failed as a result in some cases. If the start is failed once, a next starting operation can not be conducted until the engine revolution number is reduced and the rotation is stopped.
A brushless motor which does not have a position detecting sensor of a rotor is used as the starter motor in some cases. In this case, a position of the rotor is usually estimated from voltage induced in a stationary windings and a phase signal and the like. Therefore, if the supply of electricity is stopped once, the rotation speed and the rotation position can not be detected thereafter. Thus, there is a problem that if the start is failed once, the next starting operation can not be conducted until the revolution number is reduced and the engine is stopped, and it takes time for re-start.